Recently, in the medical field, it is earnestly demanded to reduce the volume of the processing solution waste from the viewpoints of environment conservation and space saving. To achieve such a demand, technologies are necessary that can produce photothermographic image-recording materials used for medical diagnosis and photographic application, which materials can be efficiently exposed with laser image-setters or laser imagers to form crisp black images of high resolution as well as superior sharpness. Such photothermographic image-recording materials eliminate the use of solution type processing chemicals and therefore can provide customers with heat development systems which are simple and environmentally friendly.
Although similar requirements exist even in the field of general image formation, it is characterized that medical images must have a high image quality inclusive of excellent sharpness and excellent granularity as fine descriptions are required, and are expected to look blue black for the ease of diagnosis. At present, there is no medical image output system that satisfies the above requirements, though a variety of hardcopy output systems are in practical use for general image formation including inkjet and electrophotographic printers which form images made of pigments or dyes.
On the other hand, heat-developable image-forming systems which use organic silver salts are known as are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075, p. 279, Chapter 9 of “Thermally Processed Silver Systems” written by D. Klosterboer and “Imaging Processes and Materials, Neblette's 8th edition”, edited by J. Sturge, V. Walworth and A. Shepp (1989). Generally, a photothermographic image-recording material has a photosensitive layer comprising a photo catalyst exemplified by silver halide in a catalytic activity quantity, a reducing agent, a reducible silver salt such as organic silver salt and, if needed, an agent for controlling the tone of the developed silver, all dispersed in a matrix of a binder. Such a photothermographic image-recording material that has been subjected to an image exposure is heated to an elevated temperature (e.g., 80° C.), causing a redox reaction between the silver halide or the reducible silver salt (acting as an oxidant) and the reducing agent to give rise to a black image made of silver. The redox reaction is promoted by the catalytic action of the latent image in the silver halide formed by the exposed light. Accordingly, a black silver image is formed at an exposed area. Photothermographic image-recording materials and systems based on the principle explained above are disclosed in many references such as U.S. Pat. No. 2,910,377 and JP-B-43-4924 (The term “JP-B” as used herein means an “examined Japanese patent publication”). A commercially available example of medical image forming systems based on such photothermographic image-recording material is given by Fuji Medical Dry Imager FM-DP L.
A photothermographic image-recording material is known in which an image-forming layer is provided by coating and drying an organic solvent-based coating solution as disclosed in U.S. Pat. No. 5,415,993. Further, for environment conservation and a higher safety, various technologies enabling the formation of the image-recording layer using aqueous solvents instead of organic solvents are proposed in, e.g., JP-A-10-10670 and JP-A-10-186565 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) . However, in the methods as described in these patents, in spite of the advantages on environment conservation and safety, the aqueous coating solutions must be kept at a pH not lower than 8 to secure a desirable coating property, as the acid coating solution for the image forming layer suffers from an undesirable viscosity increase and coagulation caused by the interaction of the binder polymer with the solid dispersions that are needed for photothermographic image-recording material. On the other hand, it is well known that the silver halide photographic emulsion in an alkaline pH environment tends to increase fog of photosensitive material prior to image formation because it becomes the reducing atmosphere. Hence, there is a problem of storing the photosensitive material prior to image formation. With such a problem in the prior art technologies, polymer materials that can impart both a good storability of image of photosensitive material prior to image formation and a desirable coating property have been urgently demanded as a binder for the image forming layer.